Recently, the number of people suffering from various cardiovascular system diseases, including heart infarction and brain infarction, has been on the rise, thus making it more and more urgent to prevent and treat these diseases.
The onset of heart or brain infarction is closely correlated to atherosclerosis. More specifically, if an atheroma is created on the arterial wall or if no arterial cells are produced anymore due to various factors such as elevated blood pressure, then the artery loses its elasticity to become hard and fragile. Also, if the blood vessel is clogged up where the atheroma has been created or if a vascular tissue covering the atheroma has ruptured, then the atheroma will move itself into the blood vessel to clog up the artery elsewhere or to rupture the hardened portions of the artery. As a result, these diseases are caused. That is why it is important to diagnose the atherosclerosis as early as possible to prevent or treat these diseases.
In the prior art, the lesion of atherosclerosis is diagnosed by directly observing the inside of the blood vessel with a vascular catheter. However, this diagnosis needs to be carried out with a vascular catheter inserted into the blood vessel of a patient, thus imposing a heavy load on him or her. For that reason, the vascular catheter observation is usually adopted to locate the lesion of atherosclerosis in a patient who is already known to suffer from that disease but has never been used to make a medical checkup on a supposedly healthy person.
A checkup may be easily made without imposing excessively heavy load on a patient if the index of cholesterol, which is one of major causes of atherosclerosis, or the blood pressure is measured. However, none of these values directly indicates the degree of advancement of atherosclerosis.
Also, if the atherosclerosis can be diagnosed early enough to administer a dedicated curative medicine to its patient, then the disease can be treated effectively. However, it is said that once the atherosclerosis has advanced to a certain degree, the farther advancement of that disease can be checked with the administration of curative medicine but it is difficult to repair the hardened artery completely.
For these reasons, a method or apparatus for diagnosing the atherosclerosis at an early stage of its advancement without imposing too much load on its patient is now in high demand.
Meanwhile, an ultrasonic diagnostic apparatus or an X-ray diagnostic apparatus has been used in the prior art as a noninvasive medical apparatus that imposes only a light load on a person under test. Specifically, by irradiating the testee with an ultrasonic wave or an x-ray that has been produced externally, shape information or information about the variation in the shape of his or her internal body with time can be acquired without causing too much pain to him or her. When the information about the variation with time (i.e., mobility information) in the shape of an object under test in his or her body can be obtained, the characteristic information of the object can be obtained. That is to say, the vascular elastic property of the organism can be known and the degree of advancement of the atherosclerosis can be detected directly.
Among other things, the ultrasonic diagnosis is superior to the X-ray diagnosis because the ultrasonic diagnosis can be made just by putting an ultrasonic probe on a person under test. That is to say, in the ultrasonic diagnosis, there is no need to administer a contrast medium to the person under test and there is no concern about potential X-ray exposure, either.
Besides, some ultrasonic diagnostic apparatuses can recently have significantly improved measuring accuracy thanks to remarkable advancement of electronic technologies. As a result, ultrasonic diagnostic apparatuses for measuring the very small movement of a vital tissue have been developed. For example, according to the technique disclosed in Patent Document No. 1, vibration components of a vascular movement, having an amplitude of several micrometers and a frequency of as high as several hundreds of Hz, can be measured accurately. Thus, it was reported that the thickness variation or strain of the vascular wall could be measured highly accurately on the order of several micrometers.
By adopting such a high-accuracy measuring technique, the two-dimensional distribution of the elastic properties of the arterial wall can be plotted in detail. For example, Non-Patent Document No. 1 shows an example of presenting the two-dimensional distribution of the moduli of elasticity of the carotid arterial wall as an image superimposed on a B-mode tomogram. The hardness of the arterial wall is not uniform but has some distribution. That is why in diagnosing the atherosclerosis, it is important to understand properly the local distribution of the moduli of elasticity, which are characteristic quantities indicating the degree of advancement of the atherosclerosis.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 10-5226    Non-Patent Document No. 1: Hiroshi Kanai et al., “Elasticity Imaging of Atheroma with Transcutaneous Ultrasound Preliminary Study”, Circulation, Vol. 107, pp. 3018-3021, 2003